Like any living organism, West Nile virus is well adapted for its own survival. It can survive in many species of birds
and can use a wide range of mosquito species as a means of transferring the virus to new individuals.
From its arrival in North America on the east coast in 1999, it quickly spread among local bird populations. Many infected
birds were migratory and carried the virus to southern climates where viral transmission continued as these birds intermingled
with local mosquitoes and overwintering birds of the same or many different species.
During spring migration, the virus was carried northward in a variety of infected birds that spread out across the continent.
Thus in 1999, the virus was restricted to the northeast U.S. By the end of 2001, it was in most states east of the Mississippi
River as well as in southern Ontario. By the end of 2004, it was found in most provinces and states.
This expansion of the West Nile virus distribution reflects the primary bird migration pathways as the virus sequentially
entered the Atlantic, Mississippi, Central, and Pacific Flyways. The virus continued to spread across North America and
now occurs throughout Canada (except British Columbia and the maritime provinces), the continental United States, Mexico,
Central America, and South America.